Flame-proof protective coating for electrical film resistors

ABSTRACT

A coating composition capable of producing coatings on resistors which do not tend to crack during and following curing comprising a prehydrolyzed tetraalkyl orthosilicate, aluminum oxide, a suspension agent, a filler and crystalline silicon dioxide having a particle size of below 149 microns, no more than 65 percent of the silicon dioxide having a particle size less than 44 microns (pass 325 mesh), and resistors coated therewith.

United States Patent Bockstie, Jr. *Dec. 4, 1973 FLAME-PROOF PROTECTIVECOATING FOR ELECTRICAL FILM RESISTORS [56] References Cited [75]Inventor: Lawrence G. Bockslie, Jr., Bradford, UNITED STATES PATENTS3,356,515 12/1967 McGlothlin 1l7/135.1 X [73] Assignee: Corning GlassWorks, Corning, 3,392,036 7/1968 McLeod 106/1 N Y 3,412,063 11/1968Jarboe et a1. 106/3835 X 3,421,909 1/1969 Rusher 106/14 1 Notice: Theportion of the term of this patent u s q en 10 A g 1938, PrimaryExaminer-Lorenzo B. Hayes has been dlsclalmed- Attorney-Sughrue,Rothwell, MiQn Zinn &Macpeak V [22] Filed: Dec. 28, 1970 211 App]. No.:102,150 [57] ABSTRACT A coating composition capable of producingcoatings Related Apphcauon Data on resistors which do not tend to crackduring and fol- Continuation-impart of $611 April lowing curingcomprising a prehydrolyzed tetraalkyl 1968 abandonedorthosilicate,aluminum oxide, a suspension agent, a filler and crystalline silicondioxide having a particle [52] 106/15 106/287 117/137, size of below 149microns, no more than 65 percent of 252/8-1 the silicon dioxide having aparticle size less than 44 [51] Int. Cl C0911 5/18 microns (pass 325mesh), and resistors coated there, [58] Field of Search 106/15 FP, 38.3,with 22 Claims, No Drawings CROSS-REFERENCES TO RELATED APPLICATIONSThis application is a continuation-in-part of US. Ser. No. 724,220 filedAprl. 25, 1968 now abandoned.

BACKGROUND OF INVENTION The present invention relates to coatingcompositions for electrical resistors, particularly film resistors. Morespecifically, the present invention relates to flame-resistant coatingsadapted for the protection of electrical film resistors against burningand smoking due to overload.

Recently, electrical film resistors have come into wide use. Normally,these film resistors comprise a substrate such as glass coated with athin film of resistor material such as tin oxide, for example. Also, itis known in the art to overcoat these film resistors with a protectivelayer. The prior art is faced with theproblem, however, that theseprotective coatings very often burn and are destroyed due to the heatresulting from severe overloadson the film resistors. This burning ofthe resistor coating not only results in the destruction of the resistoritself, but, very often, results in damage to adjacent elements in thesystem in which it is employed. The consequent damage to electricalequipment and systems from this burning of resistor coatings has led toan intensive search for a flame-proof coating,

flame-retardants to film' resistor-protective coatings.

Theseeffortsghowever, have consistently met with failure. Many of theseconventional flame-retardants were ineffective to inhibit burning at theextremely high temperatures (400-600 C) attained in film resistors underhigh overload.

Recently a protective coating for resistors has been suggestedcomprising a tetraalkyl orthosilicate, aluminumoxide, a suspensionagentand various fillers and pigments, including titanium dioxide. Althoughthis composition provides a flame-resistant coating it is subjectto thedisadvantage that, during and following ap plication and curing thecoating tends to crack thereby exposing the coated substrate.

An object of the present invention is to provide a flame-retardantcoating composition suitable for the protection of resistors.particularly, film resistors.

A further object of the present invention is to provide aflame-retardant coating composition suitable for the protection of filmresistors which will not burn or smoke at the high temperatures attainedin film resistorsxupon severe overload.

A further object of the present invention is to provide aflame-retardant coating composition suitable for the protection of filmresistors which inhibits external arcing and aids in opening theoverloaded circuit.

A further object of the invention is to provide a composition capable ofyielding coatings which do not tend to crack during and followingapplication and curing.

A further object of the present invention is to provide aflame-retardant coating composition suitable for the protection of filmresistors wherein the flame-retardant additive is compatible with theprotective coating material and does not alter the chemical ordielectric properties thereof.

BRIEF DESCRIPTION OF THE INVENTION The coating composition of thepresent invention comprises an at least partially pre-hydrolyzedtetraalkyl orthosilicate, aluminum oxide, a suspension agent, inorganicfillers and pigments and crystalline silicon dioxide. Where desired, asolvent may also be added.

Applicant has found that coatings prepared from tetraalkyl orthosilicateand aluminum oxide coating compositions may be prevented from crackingby the addition to the coating composition of a special form of silicondioxide having a certain particle size. Thus, coatings prepared from thecompositions of the present invention are not only flame-proof andarc-resistant, but are also stable against cracking.

I am aware that silica (silicon dioxide) has been previously suggestedfor incorporation in resistor coating compositions comprising atetraalkyl orthosilicate, aluminum oxide and fillers. This is not to beconfused with the present invention, however.

The silica previously suggested for addition to coating compositionssimilar to that of the present invention is amorphous, small-particlesize silica" and is incorporated in the composition for its ability tofunc tion as a suspension or thixotropic agent. These silicacompositions have a particle size in the sub-micron or colloidal rangeand are amorphous.

These silica compositions have a physical structure and particlesizewhich render them ineffective for the purposes of the present invention.These silica" compositions have no effect on the tendency of coatingcompositions to crack during and following application to resistors andcuring.

l have found that only crystalline silicon dioxide having a certainparticle size possesses the unique ability to prevent cracking of theabove-described coating compositions, i.e., wherein no more than percentof the silica has a particle size less than 44 u, the balance being inthe range 149 u to 44 u.

DETAILED DESCRIPTION OF THE INVENTION Generally, any tetraalkylorthosilicate may be employed for the purposes of the present invention.It is preferred to employ the tetralower alkyl orthosilicates such asthe methyl, ethyl, propyl, isopropyl, butyl, isobutyl, etc., esters. Itis especially preferred to employ partially hydrolyzed tetraethylorthosilicate. The degree of hydrolysis of the orthosilicate is notoverly critical. Generally, the degree of hydrolysis may vary from about10 percent to about percent. Generally, amounts of the orthosilicateranging from about 5 percent to about 55 percent based on the totalcomposition may be employed. It is to be understood that by partiallyhydrolyzed" is also meant the in situ hydrolysis of a tetraalkylorthosilicate during the coating or curing operation. Commerciallyavailable products,

i.e., Silbond H-6, as used in the Examples, available from the StaufferChemical Company, can be used as the prehydrolyzed tetraalkylorthosilicate. This contains 19% SiO by weight.

The amount of aluminum oxide employed is not always critical. At least4% Al O is required, and generally, amounts ranging from about 4 percentto about 93.9 percent may be employed.

Any conventional suspension agent may be employed for the purposes ofthe present invention. Examples of suitable suspension agents are theorganic derivatives of the various montmorillonites, etc., commonlyknown as the Bentones, e.g., Bentone 27, an alkyl amine derivative ofmagnesium montmorillonite. Additional suspension agents includeamorphous or colloidal silica, clays such as the montmorillonitesthemselves, diatomaceous earths such as kieselguhr, finely divided micaand finely divided aluminum silicate, preferably submicron in particlesize. Generally, the organic derivatives of the montmorillonites are thepreferred suspension agents. The suspension agent may be employed in anamount ranging from O.l to 2.0 percent. The suspension agent should notaffect the pH of the coating. This is important for good shelf life ofthe coating. Another suspension agent is Attagel. This also is colloidalsilica.

Any conventional inorganic filler and/or pigment material may be addedto the coating compositions of the present composition. It will beobvious that many pigments, e.g., TiO also serve as fillers. In such acase, they can be considered fillers with a coloring property. Generallyspeaking, conventional particulate nonsoluble pigments serve as fillers,and shall be considered to function as such in this invention. Thesefillers and/or pigments can be used in any desired blending ratio toform the filler and/or pigment blend and still perform their pigmentingor filling function. For instance, in the examples the pigment blend isselected to give the desired color. A preferred filler and pigmentmaterial is titanium dioxide. TiO acts as a functional filler andpigment, i.e., in addition to operating as a filler and pigment it aidsin the processing and handling of the composition by operating as asuspension agent. Generally, amounts of titanium dioxide ranging fromabout 1 percent to about 45 percent may be employed; Any additionalconventional pigments such as cobaltous aluminate (gives a blue color),Cr o (gives a green color), Fe,o,, Fe O lampblack, zinc oxide, cadmiumoxide, cadmium sulfide, cadmium .selenide, etc. may be added to thecoating composition. Generally, amounts of pigments and/or fillersbringing the total amount of the ingredients in the composition up toabout 100 percent may be added to the coating composition. The termfiller and/or pigment", as used in this application, does not includethe orthosilicate, aluminum oxide, or silicon dioxide of the presentinvention.

Sometimes fillers and pigments can serve the further function of being asuspension agent. Preferably pigments and fillers mentioned above arenot usually used as suspension agent.

The nature of the solvent employed is not overly critical. The onlyrequirement for the solvent is that it be inert with respect to theother coating ingredients. A preferred solvent is isopropanol. However,other conventional solvents which may be used include: the loweralkanols such as ethanol, propanol, etc., ketones such as methyl ethylketone, dimethyl formamide, diacetone alcohol, dimethyl sulfoxide,pyridine, furan, furfuryl alcohol, trichloroethane, N-methylpyrrolidone, iso-octane, hexane, etc. Amounts of solvent ranging from 0to 20 percent may be employed, preferably 10-20 percent.

The amount of silicon dioxide necessary for the prevention of crackingin the resulting coating ranges generally from about 1 percent to about48 percent. It is to be understood, however, that the addition of anyamount of silicon dioxide to the coating compositions of the presentinvention will have some effect on the tendency of the coating to crack.

As discussed above, I have found that the tendency of the describedcoating compositions to crack during and following curing is inhibitedonly upon the incorporation therein of certain forms of silicon dioxidehaving a specified particle size.

More specifically, l have found that employing crystalline silicondioxide having particle sizes in the range of from about 149 p. (passmesh) to 44 u (325 mesh) produces coatings which do not crack duringapplication and during or following curing, so long as no more than 65percent of the silicon dioxide has a particle size less than 44 1.. Forexample, if 66 percent of SiO has a particle size less than 44 u,cracking results. However, 100 percent of SiO can have a particle sizeequal to 44 p. or retained on 325 mesh and the object of the inventionis attained. If only 66 percent of the SiO is less than 44 y. there isslight cracking; as the percentage increases, cracking also increases.

When employing silicon dioxide wherein greater than 65 percent by weightwas of a particle size less than 44 1.4., the coatings produced werefound to have cracks. Particle sizes greater than about 149 p. tend toproduce coatings which are not easily processable and over which smoothovercoating is difficult.

Expanding upon the above, to obtain a crack, blister, pinhole and craterfree cured coating, one cannot use crystalline silicon dioxide whereingreater than 65 percent by weight of said crystalline silicon dioxide isfines, that is, crystalline silicon dioxide having a particle size lessthan 44 p. (which will pass a 325 mesh screen). Preferably no more than55 percent, most preferably 45 percent, of such fines, if present, areused.

To avoid any fines problem, one can use crystalline silicon dioxide of asingle particle size, for instance 100 mesh pass (substantially allabout 149 u or less) 200 mesh pass (substantially all about 74 p. orless) or 300 mesh pass (substantially all about 49 u or less). Thus, apreferred particle size range for the crystalline silicon dioxide is allparticles being 100 mesh (149 p.) to 300 mesh (4914.). Needless to say,any particle size distribution within this 100 to 300 mesh range (149 toE 49 p.) can be used.

However, it shall be understood it is possible to use crystallinesilicon dioxide having a particle size distribution of 149 p. down toeven I p. or smaller, so long as no more than 65 percent by weight ofsaid crystalline silicon dioxide is fines, i.e., smaller than 44 p,which passes a 325 mesh screen. See Examples VI and VII.

However, submicron sized crystalline silicon dioxide should be avoidedfor certain applications. While 1 percent by weight would not be harmfulfor most applications about 5 to 10 percent by weight of submicron sizedsilicon dioxide should be avoided. This proportion of extreme fineswould prohibit the later application of an acceptable high temperatureresistant (250 C) smooth overcoat of, e.g., a methyl phenyl polysiloxanesuch as Dow Corning 2103, available as a 60 percent resin 40 percentsolvent mixture. While most preferably no submicron silicon dioxide ispresent, many commercial sources will provide mixtures containing smallamounts of such.

Many commercial sources provide crystalline silicon dioxide which ismore inexpensive if a particle size distribution is ordered as comparedto relatively expensive crystalline silicon dioxide of one size.Accordingly, such commercial products can be used so long as they meetthe heretofore set forth criteria.

In summary, the crystalline silicon dioxide used in this invention; W W

1. must contain at least 35 percent by weight, based on the total amountof crystalline silicon dioxide, of crystalline silicon dioxide having aparticle size of from about 149 p. (100 mesh) to no smaller than 44 p.(retained on 325 mesh);

2. of the maximum 65 percent, preferab1y45 55 percent, of fines (pass325 mesh), no more than 5-10 percent; most preferably 1 percent to none,should be submicron;

3. to avoid fines problems, preferably all the crystalline silicondioxide should have a size in the range 100 mesh (149 p.) to 300 mesh 49u).

The crystalline silicas are well known in the prior art and includefused silica, silicaflour," ground silica, powdered silica, sand, etc.

In my copending application, Ser. No. 724,271, filed Apr. 25, 1968, lhave described the desirable results produced by utilizing titaniumdioxide and a suspension agent in similar coating composition which arein a noncowled condition and have a particle agglomerate size between 1and 150 microns. Briefly, the utilization of these materials greatlyincreases the potlife and enhance the processing of these compositions.

It is also disclosed therein that adjusting the alkali metal ion contentof the composition such that a coating produced therefrom contains,after curing, less than 0.02% Na, 0.01% K, 0.01% Li, 0.001% Cs and0.001% Rb, (as their oxides) enhances themoisture resistance of theresistance of the coatings.

It .is to be understood that the disclosure of application Ser. No.724,271 is incorporated herein by reference.

The compositions of the invention may be coated according to any of theconventional, well known methods: e.g., dip, spray, brush, roller, etc.Generally, sufficient material is applied to the resistor to yield afinal cured coating of between about 5 and mils, preferably 5 and 7 milsthick. It is especially preferred, although not mandatory, to apply twocoats to the resistor. The applied coating is cured by heating,preferably over an extended period of time at gradually increasingtemperatures up to about 250 C.

1n the specification and appended claims, all percentages are by weightunless otherwise indicated.

The invention will be illustrated by the following nonlimiting examples.

EXAMPLE 1 A batch of the coating composition of the present inventionwas prepared by mixing the following ingredicuts in the indicatedamounts.

Aluminum Oxide 4,540 grams EXAMPLE II A glass-tin oxide 1 K filmresistor having a rated power of 4 watts was double coated with thecomposition of Example I and cured at 250 C to yield a com bined coating10 mils thick.

No cracking of the coating either during application, curing orfollowing curing was observed.

The coated resistor was subjected to an overload of 100 times ratedpower. The resistor did not burn or smoke. Moreover, there was noexternal arcing and the circuit containing the resistor opened uponoverload.

EXAMPLE III A coating composition was prepared by mixing together thefollowing ingredients:

Aluminum oxide 4,540 grams Silicon dioxide (crystalline, 3100 mesh) 460grams Titanium dioxide 635 grams Prehydrolyzed tetraethyl orthosilicate900 grams Cab-O-Sil (amorphous submicron silica suspension agent, CabotCorp.) grams Ethanol, anhydrous 200 grams Propanol, anhydrous 272 gramsPigment (cobaltous aluminate Cr O 18:82 percent by weight) 300 grams 1 Kglass-tin oxide film resistors having a rated power of 1 watt werecoated with the above composition as in Example ll. The coating did notcrack either during or following curing or after multiple handlingoperations. Upon being subjected to a times rated power overload noburning or smoking of the coating was ob served. No external arcing wasevident and the resistor opened the circuit. a 49 [1- EXAMPLE IV Acoating composition was prepared by blending the following ingredientstogether:

Aluminum oxide 4,300 grams Silicon dioxide (crystalline, 100 mesh) 700grams Titanium dioxide 635 grams Prehydrolyzed tetrapropyl orthosilicate500 grams Prehydrolyzed tetraethyl orthosilicate 500 grams Suspensionagent (Bentone 27)-- 90 grams Isopropanol, anhydrous 472 grams Pigment(cobaltous aluminate c o,

cent by weight 300 grams 5 K glass-tin oxide film resistors having arated power of 3 watts were coated with this composition as in ExampleII. No cracking of the coating was observed during or following curing.

Upon being subjected to an overload of 100 times rated power no flaming,smoking or external arcing was 18:82 percontaining no silicon dioxide.'2 149 ,t

EXAMPLE V EXAMPLE VI The exact procedure of Examples 1 and ll wasfollowed but substituting for the 100 mesh crystalline silicon dioxidetherein the following crystalline silicon dioxide:

Particle Size (Mesh) Percent By Weight l49y.(l00) I 125 ,u. 120 96 I05;I. (140 90 74 p. 200 75 53 p. 270 55 44 ,1 325 40 and using an amountof amorphous silica as in Example lll as the suspension agent theresults obtained were equivalent to those of Examples I and II.

EXAMPLE VII The exact procedure of Examples l and II was followed butsubstituting for the 100 mesh crystalline silicon dioxide therein thefollowing crystalline silicon dioxide:

Particle Size (Mesh) Percent By Weight and using an amount of amorphoussilica as in Example III as the suspension agent the results obtainedwere equivalent to those of Examples l and II.

EXAMPLE VIII The exact procedure of Example VII was followed but 90percent of the crystalline silicon dioxide had a size less than 44 p.(pass 325 mesh), i.e., 90 percent fines. The coating cracked andblisters, craters and pinholes formed. All were visible to the eye, andthe substrate could be seen through some cracks with a microscope.

EXAMPLES IX and X Examples ll and [V were duplicated but usingkieselguhr as the suspension agent. Equivalent results were obtained.

A spectrographic Analysis of Bentone 27" gives the following result:

(All components expressed as oxides, weight percent) CuO .00l-.00l

BeO .0005-.0O5

What is claimed is:

l. A flame-retardant coating composition suitable for the protection ofresistors consisting essentially of a nonconductive, noncracking mixtureof:

a. from about 5 percent to about 55 percent of an at least partiallyhydrolyzed tetraalkyl orthosilicate binder with one-four carbon atoms ineach alkyl group thereof, the degree of hydrolysis of said orthosilicatebeing from about 10 percent to about percent;

b. from about 4 percent to about 93.9 percent aluminum oxide;

c. from about 0.1 percent to about 2 percent of a suspension agent;

d. a material selected from the group consisting of non-metallicinorganic fillers, pigments and mixtures thereof, and

e. from 1 percent to 48 percent of crystalline silicon dioxide having aparticle size in the range of less than about 149 u, no more than 65%,by weight, of the crystalline silicon dioxide having a particle sizebelow 44 11. (pass 325 mesh), which silicon dioxide prevents cracking ofthe coating composition,d) being present in an amount necessary to bringthe total amount of ingredients to 2. The composition of claim 1 whereinsaid tetraalkyl orthosilicate is tetraethyl orthosilicate.

3. The composition of claim 1 wherein said material d) includes titaniumdioxide 4. The composition of claim "I, further including from 0 to 20percent of a solvent which is inert with respect to the balance of thecoating composition ingredients.

5. The composition of claim 4 wherein said solvent is isopropanol.

6. The composition of claim 1 wherein said suspension agent is selectedfrom the group consisting of montmorillonites, amorphous silica anddiatomaceous earths.

7. The composition of claim 6 wherein element (d) is a pigment.

8. The composition of claim 7 wherein the pigment is selected from thegroup consisting of titanium dioxide, cobaltous aluminate, Cr O F8 0,,Fe O, and lampblack.

9. The composition of claim 7 wherein element (d) is TiO, in an amountof from about 1 percent to about 45 percent.

10. The composition of claim 1 wherein the suspension agent is amorphoussilica.

11. The composition of claim 1 wherein the suspension agent isdiatomaceous earth.

l2. Thecomposition as claimed in claim 1 wherein no more than 45 percentby weight of the crystalline silicon dioxide has a particle size below44 11. (pass 325 mesh).

13. The composition of claim 1 wherein said crystalline silicon dioxidehas a particle size in the range of 149 p. to about 49 ,u.

14. The composition of claim 1 wherein no more than 10 percent of saidcrystalline silicon dioxide is submicron in size.

15. The composition of claim 1 wherein said crystalline silicon dioxidehas a particle size of about 200 mesh.

16. The composition of claim 1 where the resistor is a film resistor.

17. The composition of claim 1 wherein no more than 55 percent by weightof the crystalline silicon dioxide present has a particle size below 441/. (pass 325 mesh).

18. The composition of claim 4 wherein from 10 to 20 percent of thesolvent is present.

19. The composition of claim 14 wherein no more than 10 percent of saidcrystalline silicon dioxide is submicron in size.

20. The composition of claim 1 wherein said suspension agent is selectedfrom the group consisting of clays, diatomaceous earths, mica, amorphousand colloidal silica.

21. The composition of claim 6 where element (d) is a filler.

22. The composition of claim 1 wherein no more than zero to l percent ofsaid crystalline silicon dioxide is submicron in size.

2. The composition of claim 1 wherein said tetraalkyl orthosilicate istetraethyl orthosilicate.
 3. The composition of claim 1 wherein saidmaterial d) includes titanium dioxide.
 4. The composition of claim 1,further including from 0 to 20 percent of a solvent which is inert withrespect to the balance of the coating composition ingredients.
 5. Thecomposition of claim 4 wherein said solvent is isopropanol.
 6. Thecomposition of claim 1 wherein said suspension agent is selected fromthe group consisting of montmorillonites, amorphous silica anddiatomaceous earths.
 7. The composition of claim 6 wherein element (d)is a pigment.
 8. The composition of claim 7 wherein the pigment isselected from the group consisting of titanium dioxide, cobaltousaluminate, Cr2O3, Fe2O3, Fe3O4 and lampblack.
 9. The composition ofclaim 7 wherein element (d) is TiO2 in an amount of from about 1 percentto about 45 percent.
 10. The composition of claim 1 wherein thesuspensiOn agent is amorphous silica.
 11. The composition of claim 1wherein the suspension agent is diatomaceous earth.
 12. The compositionas claimed in claim 1 wherein no more than 45 percent by weight of thecrystalline silicon dioxide has a particle size below 44 Mu (pass 325mesh).
 13. The composition of claim 1 wherein said crystalline silicondioxide has a particle size in the range of 149 Mu to about 49 Mu . 14.The composition of claim 1 wherein no more than 10 percent of saidcrystalline silicon dioxide is submicron in size.
 15. The composition ofclaim 1 wherein said crystalline silicon dioxide has a particle size ofabout 200 mesh.
 16. The composition of claim 1 where the resistor is afilm resistor.
 17. The composition of claim 1 wherein no more than 55percent by weight of the crystalline silicon dioxide present has aparticle size below 44 Mu (pass 325 mesh).
 18. The composition of claim4 wherein from 10 to 20 percent of the solvent is present.
 19. Thecomposition of claim 14 wherein no more than 10 percent of saidcrystalline silicon dioxide is submicron in size.
 20. The composition ofclaim 1 wherein said suspension agent is selected from the groupconsisting of clays, diatomaceous earths, mica, amorphous and colloidalsilica.
 21. The composition of claim 6 where element (d) is a filler.22. The composition of claim 1 wherein no more than zero to 1 percent ofsaid crystalline silicon dioxide is submicron in size.